Predict all four ABO phenotypes from heterozygous A and B parents
Explicit heterozygous parental genotypes make all four common-model phenotypes possible.
Section 17.4 · Monohybrid inheritance
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall chromosomes, genes and alleles, meiosis, haploid gametes and fertilisation restoring two chromosome sets.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which supplied parent pair can both contribute Io?
What you will learn
- Combine IAIo and IBIo gametes in a complete square
- Calculate four-category model probabilities and expected counts
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Parents | IAIo × IBIo |
| Gametes | IA or Io; and IB or Io |
| Square genotypes | IAIB, IAIo, IBIo, IoIo |
| Phenotype ratio | A:B:AB:O = 1:1:1:1 |
Combine IAIo and IBIo gametes in a complete square
A phenotype-A parent may be IAIA or IAIo, and a phenotype-B parent may be IBIB or IBIo. This lesson explicitly supplies the heterozygous genotypes IAIo and IBIo. Their normal gamete types are IA or Io from the first and IB or Io from the second. The four equal positions require every row-column combination, not just matching letter positions.
The square gives IAIB, IAIo, IBIo and IoIo. Under the common ABO key, these give AB, A, B and O respectively. Both parents can supply Io, so O is possible in this specified cross. A and B contributions can also meet in one offspring, so AB is possible. These are complete two-allele genotypes at the same gene, not four alleles in one offspring.
Calculate four-category model probabilities and expected counts
Each phenotype has probability one quarter under equal transmission and independent combination. In labelled order A:B:AB:O, the predicted ratio is 1:1:1:1. For eighty expected offspring the model gives twenty in each category. The internal square can appear in AB, A, B, O cell order; state your chosen ratio order so the labels remain clear.
The possibility of O depends on the supplied Io-carrying parental genotypes, not on the phenotype letters A and B alone. IAIA × IBIB would instead predict only IAIB and AB in the common model. A finite observed family need not contain all four possibilities. Keep the conditional parent-genotype information, expected proportions and actual records separate when interpreting a question.
Pause and explain
What phenotype does IAIB give in the common model?
Worked example
Predict labelled common-model ABO phenotype ratios for IAIo × IBIo.
Show the worked solution
- Use IA/Io from the first parent and IB/Io from the second. The four positions are IAIB, IAIo, IBIo and IoIo.
- Map these to AB, A, B and O. In A:B:AB:O order each has one of the four positions, giving 1:1:1:1.
Answer IAIo × IBIo gives IAIB, IAIo, IBIo and IoIo in equal positions; A:B:AB:O = 1:1:1:1.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Fill the square for row gametes IA, Io and column gametes IB, Io. List all genotypes and their phenotypes.
Give me a hint
Use every row-column pair.
Compare my reasoning
- Combine IA with IB and Io.
- Combine Io with IB and Io.
- Use standard allele ordering in IBIo.
IAIB → AB; IAIo → A; IBIo → B; IoIo → O.
Look for these in your work
- Four correct genotypes
- Four correct phenotype assignments
For 120 expected offspring from IAIo × IBIo, calculate A, B, AB and O counts and state the ratio in that order.
Give me a hint
Each labelled category has probability one quarter.
Compare my reasoning
- Calculate 120 × 1/4.
- Assign the result to all four categories.
Expected A 30, B 30, AB 30 and O 30; A:B:AB:O = 1:1:1:1.
Look for these in your work
- Correct thirty in every category
- Expected wording and explicit order
Compare possible O offspring from IAIo × IBIo with IAIA × IBIB. Explain why knowing only phenotypes A and B is insufficient to choose the cross prediction.
Give me a hint
Check whether both parents carry Io.
Compare my reasoning
- Identify Io in both heterozygous parents.
- Identify its absence from both homozygous parents.
- Retain phenotype-genotype ambiguity.
O is possible in IAIo × IBIo because both can supply Io. IAIA × IBIB predicts only AB and cannot give IoIo in this normal model. Phenotypes A and B alone do not establish which parental genotypes apply.
Look for these in your work
- Correct conditional O possibility
- Explains parental phenotype ambiguity
Common mistakes
- Inferring the heterozygous parents from A and B phenotypes alone
- Missing the Io-by-Io combination
- Treating four possible phenotype categories as guaranteed in four offspring
What can you explain now?
What phenotype does IAIB give in the common model?
Compare with the explanation
AB
Both IA and IB contributions are expressed.
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Make it stick
Without your notes, combine iaio and ibio gametes in a complete square and calculate four-category model probabilities and expected counts. Use the supplied allele key and state what the evidence can establish.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.